What conversations should these communities have together, and which should they have alone?

One proposal could be to keep intact all WikiProjects, but have forums redirect to one central location. If any WikiProject begins to cause a lot of specialized comments to go into the general forum then that project's community can go into their own forum.

Do people from the different communities want to talk to each other? Start with OA, Linux/OSS, OER to find out if any pushback - then reach out to others